Noble Gases in the Atmosphere

Abstract

The atmosphere is the primary terrestrial reservoir of the heavy noble gases (Ne, Ar, Kr, Xe) and precise knowledge of the isotopic composition of atmospheric noble gases is important for many—if not all—fields of noble gas geochemistry. Air noble gases, including helium, are very commonly used as a running laboratory standard for calibrating instrumental discrimination and sensitivity (see Chap. 1), hence any potential temporal or spatial heterogeneities in the atmospheric noble gas composition could have consequences for the reliability and comparability of noble gas data. Metrological measurements such as the determination of Avogadro’s constant and the gas constant also depend on accurate determination of the isotopic composition (and isotopic masses) of atmospheric noble gases. However, absolute isotopic measurements are not straightforward and this section reviews both how absolute isotopic determinations have been made and assesses the temporal and spatial variability of the atmosphere at the present and in the recent (<2 Ka) past.